dual n back and chess

although it may sound a little bit unrealistic i belive in numbers and i play chess like since 10 years although i have never put a serious effort to improve it i play frecuently mostly 5 min blitz, last years i was here around 1900 rating flucuating between 1800 to 1900, i got 1950 to 2000 at my maximum strengh, then i leave chess for a time because i have reached my 2k elo then i back a couple of weeks ago, but in between i did a lot of dual n back and study said that it improve working memory and fluid intelligence i has a lot of free time so i tried it doing 20 sesions a day for 300 days some of them i did like 40 sesions a days, the study say that it impreve like 10 iq points doing it 20 sesions a day per 20 days i did 300 days but i dont know at some point i reach my peak and i cannot improve anymore my maximum was dual 10 back althoug it seems to perdurated in the time i still can reach 10 like i did some days ago, the fact is after leave chess do 300 days dual n back now i have return and i improve my blitz rating here like 100 to 200 points, now my confort zone is around 2050 and i have reach 2098 its like a year ago  i cant and i hardly did to reach 2k and now 2000 feels easy

What was your strategy? did you use intuition or a kind of subvocalization while doing dual n back?